1. Field
The present application for patent relates generally to multi-carrier wireless communication systems, and more specifically to reverse link power throttling.
2. Background
Communication systems 100 may use a single carrier frequency or multiple carrier frequencies. In wireless communication systems 100, the forward link refers to communications from the access network (AN) 120 to the remote station 106 (or access terminal 106), while reverse link refers to communications from the remote station 106 to the network 120. (The AT 106 is also known as a remote station, a mobile station or a subscriber station. Also, the access terminal (AT) 106, may be mobile or stationary). Each link may incorporate a different number of carrier frequencies. An example of a cellular communication system 100 is shown in FIG. 1A where reference numerals 102A to 102G refer to cells, reference numerals 160A to 160G refer to base stations and reference numerals 106A to 106G refer to access terminals.
The communication system 100 may be a code division multiple access (CDMA) system having a high data rate (HDR) overlay system, such as specified in the HDR standard. The AN 120 communicates with an AT, as well as any other ATs 106 within system, by way of the air interface. The AN 120 includes multiple sectors, wherein each sector provides at least one channel. A channel is defined as the set of communication links for transmissions between the AN 120 and the ATs 106 within a given frequency assignment. A channel consists of a forward link and a reverse link.
A high data rate subscriber station 106 may communicate with one or more HDR base stations 160, referred to herein as modem pool transceivers (MPTs) 160 via an air interface. An access terminal 106 transmits and receives data packets through one or more modem pool transceivers 160 to an HDR base station controller 130, referred to herein as a modem pool controller (MPC) 130. Modem pool transceivers 160 and modem pool controllers 130 are parts of the access network (AN) 120. The access network 120 may be further connected to additional networks 104 outside the access network 120, such as a corporate intranet or the Internet, and may transport data packets between each access terminal 106 and such outside networks. An access terminal 106 that has established an active traffic channel connection with one or more modem pool transceivers 160 is called an active access terminal 106, and is said to be in a traffic state. An access terminal 106 that is in the process of establishing an active traffic channel connection with one or more modem pool transceivers 130 is said to be in a connection setup state. An access terminal 106 may be any data device that communicates through a wireless channel or through a wired channel, for example using fiber optic or coaxial cables. An access terminal 106 may further be any of a number of types of devices including but not limited to PC card, compact flash, external or internal modem, or wireless or wireline phone. The communication link through which the access terminal 106 sends signals to the modem pool transceiver 160 is called a reverse link. The communication link through which a modem pool transceiver 160 sends signals to an access terminal 106 is called a forward link.
FIG. 1B is a simplified functional block diagrams of an exemplary CDMA communications system. A base station controller 130 can be used to provide an interface between a network 104 and all base stations 160 dispersed throughout a geographic region. For ease of explanation, only one base station 160 is shown. The geographic region is generally subdivided into smaller regions known as cells 102. Each base station 160 is configured to serve all subscriber stations 106 in its respective cell. In some high traffic applications, the cell 102 may be divided into sectors with a base station 160 serving each sector. In the described exemplary embodiment, three subscriber stations 106A-C are shown in communication with the base station 160. Each subscriber station 106A-C may access the network 104, or communicate with other subscriber stations 106, through one or more base stations 160 under control of the base station controller 130.
Modern communications systems are designed to allow multiple users to access a common communications medium. Numerous multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), space division multiple-access, polarization division multiple-access, code division multiple-access (CDMA), and other similar multi-access techniques. The multiple-access concept is a channel allocation methodology which allows multiple user access to a common communications link. The channel allocations can take on various forms depending on the specific multi-access technique. By way of example, in FDMA systems, the total frequency spectrum is divided into a number of smaller sub-bands and each user is given its own sub-band to access the communications link. Alternatively, in TDMA systems, each user is given the entire frequency spectrum during periodically recurring time slots. In CDMA systems, each user is given the entire frequency spectrum for all of the time but distinguishes its transmission through the use of a code.
In multi-access communications systems, techniques to reduce mutual interference between multiple users are often utilized to increase user capacity. By way of example, power control techniques can be employed to limit the transmission power of each user to that necessary to achieve a desired quality of service. This approach ensures that each user transmits only the minimum power necessary, but no higher, thereby making the smallest possible contribution to the total noise seen by other users. These power control techniques may become more complex in multi-access communications systems supporting users with multiple channel capability. In addition to limiting the transmission power of the user, the allocated power should be balanced between the multiple channels in a way that optimizes performance.
A power control system can be employed to reduce mutual interference between the multiple subscriber stations 106. The power control system can be used to limit the transmission power over both the forward and reverse links to achieve a desired quality of service. For the purposes of illustration, the gain computation techniques will be described with reference to the reverse link, however, as those skilled in the art will readily appreciate, these gain computation techniques are equally applicable to the forward link.
The reverse link transmission power is typically controlled with two power control loops. The first power control loop is an open control loop. The open control loop is designed to control the reverse link transmission power as a function of path loss, the effect of base station 160 loading, and environmentally induced phenomena such as fast fading and shadowing. This open control loop estimation process is well known in CDMA communications systems.
The second power control loop is a closed control loop. The closed control loop has the function of correcting the open loop estimate to achieve a desired signal-to-noise ratio (SNR) at the base station 160. This can be achieved by measuring the reverse link transmission power at the base station 160 and providing feedback to the subscriber station 106 to adjust the reverse link transmission power. The feedback signal can be in the form of a reverse power control (RPC) command which is generated by comparing the measured reverse link transmission power at the base station 160 with a power control set point. If the measured reverse link transmission power is below the set point, then an RPC up command is provided to the subscriber station 106 to increase the reverse link transmission power. If the measured reverse link transmission power is above the set point, then an RPC down command is provided to the subscriber station 106 to decrease the reverse link transmission power.
The open and closed control loops may be used to control the transmission power of various reverse link channel structures. By way of example, in some CDMA communications systems, the reverse link waveform includes a traffic channel to carry voice and data services to the base station 160 and a pilot channel used by the base station 160 for coherent demodulation of the voice and data. In these systems, the open and closed control loops can be used to control the reverse link power of the pilot channel. In order to optimize performance, the power of the pilot channel can then be balanced with the power of the traffic channel. Specifically, each channel can be spread with a unique orthogonal code generated by using Walsh functions. A gain can then be applied to the traffic channel in order to maintain an optimal traffic to pilot channel power ratio.
This principle can be extended to additional channels in the reverse link waveform. In CDMA communications systems with a variable data rate, for example, a data rate control (DRC) channel containing a DRC message is generally supported by the reverse link transmission. In the variable data rate mode, the data rate of the forward link transmission is dictated by the DRC message. The DRC message is typically based on a carrier-to-interference (C/I) estimation performed at the subscriber station 106. This approach provides a mechanism for the base station 160 to efficiently transmit the forward link data at the highest possible rate. An exemplary CDMA communications system supporting a variable data rate request scheme is a high data rate (HDR) communications system. The HDR communications system is typically designed to conform one or more standards such as the “cdma2000 High Rate Packet Data Air Interface Specification,” 3GPP2 C.S0024, Version 2, Oct. 27, 2000, promulgated by a consortium called “3.sup.rd Generation Partnership Project.”
Open and closed control loops may be used to control the transmission power of various reverse link channel structures. For example, U.S. Pat. No. 6,594,501, entitled “Systems and Techniques for Channel Gain Computations,” discloses throttling or backing off predetermined power ratios of the data rate control channels (DRC) and the acknowledgement (ACK) channels with respect to the pilot channel if the total reverse link transmission power exceeds the maximum power capability of the transmitter.